Frequently used commercial methods for producing clearcoats for automotive and other coatings applications involve application of a clearcoat composition over a high-solids basecoat. Clearcoat compositions can be characterized as being of the one-component type or the two-component type. In the two-component or "two-pack" system, the polymer and the cross-linking agent are combined Just prior to coating. However, this two-component process generally requires mixing of the components in the right proportion immediately prior to coating, and is expensive to operate and difficult to control. In contrast, the one-component or "one-pack" systems offer significant advantages in manufacturability because the cross-linkable clearcoat coating composition can be coated as one formulation. However, when highly reactive crosslinking agents such as polyisocyanates are used, the cross-linking agent in one-component systems must be blocked in order to prevent premature cross-linking of the clearcoat composition. The blocking group can then be unblocked under specified conditions, such as an elevated temperature, to allow the materials to crosslink so the coating can be cured.
Up until recently, an elevated temperature of 160.degree. C. or more was required to cause unblocking of the polyisocyanate in order to enable the desired clear-coat to form during the coating operation. Recently, there has been a trend in the industry to discover blocking agents that unblock at lower temperatures, thereby affording an energy savings during production of the desired coating, regardless of whether a clear-coat or a pigmented paint is desired. Illustrative examples of blocking agents that provide unblocking at lower temperatures are the pyrazole-containing compounds as disclosed in U.S. Pat. No. 4,976,837 for use in pigmented paint compositions.
Clearcoats that have been cross-linked with blocked isocyanate cross-linking agents also exhibit good hardness, and they are also resistant to environmental etch, i.e., etch appearing as milky or cloudy marks on clearcoat finishes that have been exposed to the elements. Unfortunately, however, these clearcoats suffer from severe yellowing during heat curing. Moreover, unlike coatings derived from unblocked aromatic polyisocyanates, coatings derived from blocked isocyanates tend to exhibit significant yellowing during cure even when an aliphatic polyisocyanate is used.
Heretofore, attempts to reduce the yellow discoloration of clearcoats that have been cross-linked with blocked isocyanates have typically utilized additives to inhibit or otherwise minimize the yellow discoloration problem. By way of illustration, U.S. Pat. No. 5,216,078 discloses the addition of a hydrazide group, either attached to the blocked polyisocyanate itself, or attached to the polyisocyanate-reactive polymer (e.g., polyol) in the formulation, or attached to another compound as a formulation additive. Unfortunately, the introduction of extraneous hydrazide moieties to the clearcoat-forming formulation can adversely affect the formulation and/or processing expense of the resulting clearcoat.
In view of the above, new methodology for providing a one-component polyisocyanate-cross-linked clear coating which avoids discoloration without the use of discoloration-prevention additives would be highly desired by the clear coatings manufacturing community.